This invention relates to a large area photovoltaic device, such as a solar cell, configured so as to provide a consistently high level of performance despite the presence of shorts and shunts which may occur to the cell during processing, handling or use. Generally, the invention contemplates the concept of dividing the large area cell into a plurality of groups of discrete, isolated, series-connected, small area photovoltaic segments, which groups are then electrically connected in parallel. In this manner defects in, or damage to, a given small area segment of the large area photovoltaic cell will not result in a disproportionate loss in the electrical performance of the large area cell, since the effect of the defect or damage will be restricted to the discrete small area segment in which it occurs. The loss of electrical performance in a series-parallel connected large area cell will only be fractional and proportional to the loss in surface area of the damaged small area segment. The invention includes a method for the manufacture of these large area photovoltaic cells from deposited thin film layers of semiconductor material, said method of manufacture being readily adaptable to a continuous process.
Owing to the increasing world demand for energy and the decreasing availability of nonrenewable energy sources such as fossil fuels, photovoltaic power has become increasingly attractive. Initially, sources of photovoltaic power were restricted to single crystal photovoltaic materials, which materials are difficult to fabricate, expensive to manufacture and of limited size. Consequently, crystalline photovoltaic materials are of limited utility for general power generating applications. Recently, a number of technologies have been developed for manufacturing thin film photovoltaic devices from such materials as cadmium sulfide, cadmium telluride, gallium arsenide, gallium aluminum arsenide, amorphous silicon alloys, amorphous germanium alloys and the like.
Because of their economical manufacture and ease of processing, thin film materials are replacing single crystal materials in photovoltaic applications. Thin film photovoltaic cells generally cost far less to produce than single crystal cells. Single crystal materials must be grown by a time consuming, energy intensive process, whereas thin film materials may be rapidly and economically deposited via vacuum deposition processes, plating processes, chemical vapor deposition processes and the like. Furthermore, the process of fabricating single crystal photovoltaic cells is wasteful of material, i.e., before the cells can be utilized, large single crystal ignots must be physically cut into wafers. In contrast thereto, thin film photovoltaic materials may be directly deposited onto a substrate, so as to form a photovoltaic cell, in a deposition process which fully utilizes all of the deposited semiconductor material. The flexibility of thin film photovoltaic materials is another reason that such materials are very attractive for use in photovoltaic cells. These materials may readily be configured to cover large areas, whereas single crystal materials such as silicon can only be grown into 6 inch diameter wafers. And since, thin film materials are flexible, photovoltaic cells capable of conforming to a variety of geometries may be manufactured. As is obvious from the foregoing, photovoltaic energy sources are of increasing importance, and thin film photovoltaic cells have unique properties making them specially suited for the economical, wide-spread generation of electrical power.
The active semiconductor layers of thin film photovoltaic cells are, by definition, quite thin, i.e., approximately several hundred to several thousand angstroms thick. While the materials utilized in thin film, large area photovoltaic devices, especially amorphous silicon alloy materials, are quite durable, the extreme thinness thereof may make small area segments of the large area device susceptible to damage such as the formation of low resistance current paths, which, in the absence of proper photovoltaic device design, could disproportionately reduce the performance of the large area device.
Defects in a thin film device can take the form of shorts or shunts which, as mentioned, provide a low resistance current path through the thin film semiconductor material. As is well known to those skilled in the art, even a small number of such low resistance current paths can effectively short circuit an entire photovoltaic cell, thereby degrading or destroying its electrical performance. Shorts and shunts can arise during the fabrication of a photovoltaic cell if, for example, (1) a defect such as a jagged spike or protuberance is present upon the surface of the substrate atop of which the thin film layer of semiconductor material is deposited, or (2) if the deposited thin film layer does not completely cover the substrate layer therebelow. While it may be theoretically possible to eliminate all low resistance current paths during processing, the economics and realities of producing defect-free large area photovoltaic devices demands the manufacture of a large area device which is tolerant of such defects. Furthermore, low resistance current paths occurring after manufacture of the photovoltaic device will also deleteriously affect the performance of that device if the device is not defect-tolerant. Additionally, note that damage may occur to the small area segments of the large area photovoltaic device while in situ, if, per chance, they are punctured by hail, rocks, bullets, or other projectiles. While projectile impact is likely to shatter a single crystal photovoltaic device, thereby rendering it inoperative, damage to thin film photovoltaic devices will generally be limited to a puncture in a small area segment thereof. In some cases the puncture will cause no damage whatsoever, in other cases it may create a low resistance current path, either immediately, or over a course of time owing to degradation at the puncture site by the action of moisture and oxygen. Obviously, it is highly desirable to have a photovoltaic cell which is (1) insensitive to, or tolerant of, the effects of low resistance current paths, and (2) resistant to puncture damage.
As discussed hereinafter, the effects of shorts and shunts may be mitigated or intensified, depending upon the current collection design of the large area device. More particularly, if a large area semiconductor body was designated to form a single, unsegmented large area photovoltaic cell, the presence of a single low resistance current path could disproportionately destroy the electrical output of the entire large area cell. Obviously, the probability of damaging the entire electrical output of a large area segmented photovoltaic cell is dependent upon the surface area of that large area cell, the number of small area segments into which it is divided, and the manner in which those small area segments are electrically connected. It is for this reason that large area photovoltaic cells are divided into a plurality of small area segments. It is to be noted at this point that the term "cell", as used herein, will refer to a unitary, photovoltaic device disposed upon a single discrete substrate. A large area cell may therefore include a plurality of electrically interconnected, small area photovoltaic segments, each capable of producing an electrical current in response to illumination, said small area segments cooperating to provide the total electrical output of the large area photovoltaic cell. The term "photovoltaic segment", as used herein, will refer to one of the small area subcells, or individual subunits into which the large area photovoltaic cell is divided.
If the small area segments into which the large area photovoltaic cell may be subdivided are electrically connected in series, the sum of the voltages of the individual small area segments produce the voltage of the large area photovoltaic cell. In such a series connected arrangement of small area segments, an individual shorted or shunted segment will not necessarily destroy the operation of the entire large area cell since electrical current will merely flow through the low resistance current path to the next series connected small area segment. The utility of dividing a large area cell into a plurality of series connected small area segments is limited by the fact that the voltage is additive. The voltage of the series connected small area segments is dependent upon the number of functioning small area segments of the large area cell. If a large number of small area segments are inoperative, the sum of the voltages of the entire large area cell may be severely reduced. Furthermore, since the output voltage of a large area photovoltaic cell having series connected small area segments is dependent upon the number of operative small area segments, damage occurring after the large area cell is installed may also cause severe voltage loss in that large area cell. A large area photovoltaic cell which is divided into a very large number of series electrically interconnected small area segments will not have its output voltage changed significantly by the loss of a few segments; however such a device would exhibit a high output voltage and a low output current, and thereby be of limited utility.
The use of parallel connected small area segments is an alternative approach to the division of a large area photovoltaic cell into a plurality of small area segments, which connection will prevent a small number of defective portions thereof from destroying the electrical output of the entire cell. A parallel connected array of small area photovoltaic segments will provide a relatively high output current at a voltage equivalent to that of a single small area segment, i.e. the photogenerated currents of the plurality of small area segments are additive. The use of a parallel connected array of small area segments therefore offers the advantage of a high output current. However, a major problem exists with utilizing a parallel connected array of small area segments because a single shorted or shunted small area photovoltaic segment can effectively short circuit the entire large area photovoltaic cell by providing a low resistance current path in parallel with the small area segments.
One solution to the problem of having defective small area segments in a parallel connected array of those small area photovoltaic segments, into which a large area solar cell is divided, destroy the electrical output of that entire large area cell is disclosed in U.S. Pat. No. 4,419,530 of Prem Nath, entitled IMPROVED SOLAR CELL AND METHOD FOR PRODUCING SAME, which patent is assigned to the assignee of the instant invention and the disclosure of which is incorporated herein by reference. The method described by the Nath patent includes the steps of subdividing a large area photovoltaic device into a plurality of discrete, electrically isolated small area photovoltaic segments and electrically connecting, in parallel, only those small area segments found to be electrically operable (i.e. free of defects and capable of producing an adequate electrical output). The Nath invention is therefore capable of reducing electrical power losses normally experienced when individual small area segments of a large area photovoltaic cell are defective. However, subsequent damage to one or more of the small area segments of the large area photovoltiac cell, which damage can short circuit the small area segment, will still detrimentally affect the performance of the entire large area photovoltaic cell. Furthermore, the method disclosed by Nath, requires the individual testing of each small area segment before it is incorporated into the parallel connected array which defines the large area cell. Obviously, such a testing procedure is time and labor intensive, and represents an additional expense in the fabrication of large area, thin film photovoltaic cells. Accordingly, the need still exists for a thin film, large area photovoltaic cell having an electrical output which is insensitive to the effects of low resistance current paths and/or other damage occurring either prior to, during, or after cell fabrication.
A combined series-parallel array of small area photovoltaic segments has previously been employed to form a thin film, large area photovoltaic cell, thereby overcoming the problems previously discussed with reference to series connected and to parallel connected small area segments thereof. According to known principles, a plurality of small area photovoltaic segments may be arrayed in rows and columns upon a substrate. The small area photovoltaic segments within each given row are electrically connected in series so as to form a series connected row, and those series connected rows are then electrically connected in parallel, thereby forming the large area photovoltaic cell. It should be noted that the terms "rows" and "columns" as used herein, will designate subgroups of the small area segments within the matrix array of the small area segments which combine to form the large area photovoltaic cell. The term "rows" and "columns" as used herein include any grouping of adjacent small area segments, and accordingly may be used interchangably; for example, the series connected segments may be described as series connected rows or series connected columns. However, solely for the sake of consistency and clarity, the series connected small area segments will be described in this specification as being aligned in parallel rows, it being understood that they could equally well be described as being aligned in columns. Furthermore, it is not necessary that the rows and columns be oriented at right angles to one another; other arrangements such as diagonal arrangements may be similarly employed. In summary, the series-parallel arrangement is defined to include groups of series connected photovoltaic segments, generally referred to herein as rows, which rows are then electrically connected in parallel.
It should thus be apparent that the output voltage of a series-parallel arrangement of the small area segments of a large area photovoltaic cell will be determined by the voltage produced in each series connected row, while the total output current will be determined by the sum of the current produced by each of the parallel connected rows. Thus as is also readily apparent, the electrical characteristics of a large area photovoltaic cell may be selected by choosing the appropriate series-parallel arrangement of the small area segments thereof.
The importance of such a series-parallel connected large area photovoltaic cell is that it is resistant to the effects of shorts, shunts, low resistance current paths and other mechanical damage to individual small area segments. For example, if an individual small area segment develops a current shunting type of defect, it will create a low resistance current path between the two adjacent small area segments with which it is electrically connected in series. Such current shunting defect will not short circuit current from the entire large area cell as would be the case if the small area segments were connected in parallel. Although the single defective small area segment will proportionally lower the output voltage of the row of small area segments with which it is connected in series, the effect of the single defective small area segment upon the total voltage output of the large area photovoltaic cell will be negligible since there are a plurality of rows of such series connected small area segments, the rows connected in parallel. Therefore, the fractional loss in electrical performance exhibited by the large area photovoltaic cell will be equal to the number of current shunting small area segments divided by the total number of small area segments into which the large area photovoltaic cell is divided. On the other hand, damage to a small area segment of the large area photovoltaic cell which results in a complete loss of the electrical output of that small area segment (i.e. an "open circuit" rather than a short circuit), destroys the electrical output of the entire row in which that small area segment is electrically connected in series, but does not (1) impair the electrical operation of the remaining rows of small area segments of the large area cell, or (2) lower the operating voltage of the large area cell. In contradistinction, a completely series connected array of small area segments, in which an open circuit condition exists, renders the large area photovoltaic cell inoperative. According to the principles of this invention, the individual small area segments may be fashioned of such a small size that the need for grid patterns thereon is obviated since the current path across any given small area segment is quite short. It should thus be apparent from the foregoing description that the utilization of a series-parallel arrangement of small area segments in the fabrication of large area photovoltaic cells is desirable.
U.S. Pat. Ser. No. 4,315,096 of Tyan, et al, entitled INTEGRATED ARRAY OF PHOTOVOLTAIC CELLS HAVING MINIMIZED SHORTING LOSSES, discloses one such series-parallel connected array of small area segments of a large area photovoltaic cell, said cell formed by a plurality of discrete semiconductor members, the number of which corresponding to the number of small area photovoltaic segments into which the large area cell is divided. The semiconductor members are electrically connected by electrodes which are configured to form the preselected series-parallel arrangement. While Tyan, et al, provides a series-parallel connected large area photovoltaic cell, the method disclosed therein is cumbersome insofar as it necessitates the use of a large number of discrete semiconductor members which must be either individually deposited by a series of masking and deposition steps, or formed from a continuous layer of semiconductor material which is subsequently scribed. It should also be noted that Tyan et al., offers no solution to the problems (they do not even acknowledge the existence of the problems) resulting from puncture damage to the large area photovoltaic cell after installation. Even if the large area cell enjoys a series-parallel arrangement of small area segments, the effects of such damage on overall cell performance may be severe if oxygen and moisture from the ambient atmosphere enter and corrode portions of the large area cell. Obviously, it is desirable to eliminate the steps disclosed and required by the method of Tyan et al in order to effect their series-parallel electrical connection, and thereby effect savings in time and cost. Furthermore, there still exists a need for a large area photovoltaic cell capable of sustaining puncture damage without severely and deleteriously affecting the overall electrical output of that cell.
The instant invention provides a large area photovoltaic cell divided into a series-parallel connected array of small area photovoltaic segments, and a method of fabricating such a large area cell. The large area cell, so fabricated, does not require the use of individual semiconductor members to form each of the small area segments, but rather, a single elongated semiconductor member or "bridge" electrically contacts a plurality of electrode members within a given column. In this manner, the number of semiconductor bridges utilized to fabricate the large area photovoltaic cell is reduced, the number of semiconductor bridges being equal to the number of small area photovoltaic segments within a given row. The improved large area photovoltaic cell design, described herein, results in a savings in the number of processing steps, of processing time, and cost, and reduces the likelihood of damage to the small area segments during the electrical connection thereof. The instant invention also provides a large area photovoltaic cell capable of sealing punctures resulting from the impact of projectiles which might occur after installation, so as to prevent degradation of the exposed semiconductor material due to exposure to ambient conditions. The instant invention thereby significantly reduces problems which have heretofore limited the efficient and widespread use of thin film, large area photovoltaic devices.
It should therefore be apparent that the instant invention is well suited for use with any type of thin film photovoltaic device, and has special utility in the fabrication of thin film, large area photovoltaic cells which incorporate amorphous semiconductor alloy layers.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloy materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge deposition or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374 of Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors; and U.S. patent application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy. As disclosed in these patents and application, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E.D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in U.S. Pat. No. 4,400,409 for a Method of Making P-Doped Silicon Films; U.S. Pat. No. 4,410,588 for Continuous Amorphous Solar Cell Production System, and in pending patent applications: Ser. No. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,438,723, for Multiple Chamber Deposition And Isolation System And Method; U.S. Pat. No. 4,492,181, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and U.S. Pat. No. 4,485,125, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these patents and patent applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained. It is for the production of large area photovoltaic cells fabricated from the materials and by the processes enumerated hereinabove, that the series-parallel connections including semiconductor bridging of the small area segments described by the instant invention may be utilized. Employing the techniques referred to in the patents and applications referenced above, and the teachings of the instant invention, a novel, continuous process for the manufacture of improved, damage resistant, large area photovoltaic cells is disclosed.